The susceptibility of an organism to a drug is determined by

The susceptibility of an organism to a drug is determined by size of the zone of inhibition that is dependent on certain factors/variables. What factors name at least four may affect the size of the zone of inhibition.

Solution

Answer:

Factors that may affect the size of the zone of inhibition are:

(1) Pathogen susceptibility:

The antibiotic to be used in the inhibition plate depends upon the microorganism under study. If the organism is resistant to the antibiotic, it will grow near to the discs while if the organism is susceptible, it will not grow around the disc creating a zone of inhibition.

(2) Size of the inoculum:

Size of the inoculum is also an important factor. If size of the inoculum is large, the zone of inhibition will be small while if size of the inoculum is small, the diameter of the zone of inhibition will be large.

(3) Antibiotic diffusion:

Antibiotics differ in their solubility, molecular weight and size. So the diffusion rate of different antibiotics are different in agar media. Large sized molecules shall diffuse slowly compared to the smaller ones. These factors together may affect the zones of inhibition.

(4) Agar depth in plates:

The depth of agar also determines the size of the zone. For example, if the agar media have benn prepared from dehydrated media, the inhibition plates should be poured to a 4mm depth. If the media is too shallow, the antibiotic will diffuse further and produce larger zones of inhibition and give false positive results. Similarly, too low media level will show smaller zones, thus denoting resistance.